1. Field of the Invention
The present invention relates to a printing device and method for forming dots during the movement of a head as it travels back and forth to print multi-colored multi-tone images on a printing medium, and to a recording medium on which is recorded a program for such printing.
2. Description of the Related Art
Various printers have been used in the past as computer or digital camera output devices. Such printers include ink jet printers that jet ink to form dots and print multi-colored multi-tone images. In ink jet printers, dots are formed for each pixel by repeated primary scanning, in which the head travels back and forth, and sub-scanning, in which the printing paper is conveyed. Dots are formed by ink of predetermined colors, and multiple colors are brought out by the overlapping of these inks. The tones of images are brought out by the dot recording density.
Ink jet printers commonly make use of multinozzles comprising a plurality of nozzles arranged at a constant pitch in the sub-scanning direction for each color in order to enhance printing speed. In such cases, differences in the ink discharge properties of each nozzle can cause shifts in the positions where the dots are formed. Feed errors during sub-scanning can also cause shifts in the positions where the dots are formed. Such shifts can cause irregularities in density, referred to as banding, which can result in a loss of image quality. Printing based on what is referred to as interlacing or overlapping formats has been proposed in an effort to suppress such loss of image quality due to banding.
Interlacing refers to a format for printing images as raster lines are intermittently formed in the sub-scanning direction. FIG. 20 illustrates an example of the interlacing format. This is a case involving the use of 3 nozzles arranged at a nozzle pitch k of 2 dots. In FIG. 20, the circles containing 2 digits indicate the dot recording positions. The left numeral of the two-digit numbers indicates the nozzle number, and the right number indicates the primary scan during which the dot is printed.
The dots of each raster line are formed by the 2nd and 3rd nozzles in the first primary scan in the interlaced format recording illustrated in FIG. 20. The first nozzle does not form dots. After paper feed L equal to three raster lines, each raster line is formed using the first through third nozzles in the second primary scan. Images are subsequently printed by similarly repeating paper feed equal to three raster lines, and raster line formation by primary scanning. No raster lines are formed by the first nozzle in the first primary scan because no adjacent raster lines can be formed by second and subsequent primary scanning under the raster lines.
The overlapping format refers to the formation of raster lines with two or more nozzles by intermittently forming dots on the raster lines in each primary scan. For example, in the first primary scan, odd-numbered pixels of a given raster line are formed with one nozzle, and in the second primary scan, the even-numbered pixels are formed by another nozzle. Raster lines can also be formed by 3 or more scans, of course.
Shifts in the dot formation position due to sub-scan feed errors or ink discharge properties during interlacing or overlapping format printing can be dispersed in the sub-scanning direction or primary scanning direction. Shifts in the dot forming position can thus be rendered negligible, banding can be suppressed, and image quality can be improved.
Better image quality as well as faster printing are also generally important in improving printer convenience. A technique for forming dots during the movement back and forth in primary scanning has been proposed in order to improve printing speed in ink jet printers (such printing is henceforth referred to as bi-directional printing). A combination of the interlaced or overlapping formats of printing with bi-directional printing enables faster printing with better image quality in ink jet printers.
In bi-directional printing, however, the positions where the dots are formed can sometimes shift in the primary scanning direction for various reasons, such as backlash in the mechanisms moving the head back and forth or errors in the head position detection. There is a need to set the primary scanning direction for forming pixels by taking into account the effects of such shifting on image quality in order to obtain good image quality during bi-directional printing.
The printing device in JAPANEZE PATENT LAID-OPEN GAZETTE No. 7-251513 is an example of the study of such matters. This printing device involves the use of a head including a plurality of nozzles at a pitch of 2 dots in the sub-scanning direction. An example of bi-directional printing employing the overlapping format to form raster lines with two nozzles has also been disclosed as an enhanced printing mode. According to this disclosure, good text quality is achieved in the first mode, where the even-numbered pixels of the raster lines are formed during forward travel in primary scanning, and the odd-numbered pixels are formed during return travel of primary scanning. Good image quality with solid ink and no drop out is achieved in the second mode, where the even-numbered raster lines are formed during forward travel in primary scanning, and the odd-numbered raster lines are formed during return travel in primary scanning.
However, this is only an extremely limited study, the object of which is merely a head with nozzles arranged at a pitch of 2 dots. A head with nozzles arranged at a pitch of 2 dots affords only three modesxe2x80x94the above two described modes and another mode in which pixels formed during movement in the same direction are disposed in a checkered pattern. The above document studies the relation of image quality in two out of the three modes.
The resolution of ink jet printers has been developed to an extremely high degree in recent years, with a trend toward the use of finer dots. Because of manufacturing limitations, the head nozzle pitch is often greater than 2 dots. A head nozzle pitch greater than 2 dots is also desirable to open up the interval in the sub-scanning direction of the dots formed in one primary scan and to prevent the dots from smearing. The correlation between the pixels and the direction in primary scanning is more diverse with the use of heads in which the nozzles are arranged at a pitch greater than 2 dots.
In such cases, there are no conventional examples studying whether pixels should be formed during forward or return travel to improve image quality. In other words, there is room for further improvement in image quality in conventional printer devices by improving the correlation between the direction of movement during the formation of the pixels.
An object of the present invention is to improve image quality in bi-directional printing, and also to provide a technique for faster and higher resolution printing.
A printing device of the present invention prints multi-colored images by means of primary scanning and sub-scanning so as to form dots for pixels on a printing medium. During the primary scanning, a head travels back and forth relative to the printing medium to form raster lines. During the sub-scanning, the printing medium is conveyed relative to the head in the direction across to the primary scanning direction. The head includes, in the sub-scanning direction, at intervals of two or more raster lines per color, a plurality of nozzles for discharging ink. And the printing device includes: memory for storing control parameters, including the position of the pixels that are to be formed during each primary scan and the feed of the sub-scan; head drive controller for driving the head while moving back and forth in the primary scanning to form dots for the pixels specified by the control parameters; and sub-scanning mechanism for effecting sub-scanning at a feed specified by the control parameters. The parameters are set in compliance with conditions allowing the direction of the primary scan during the recording of dots with each ink to be locally aligned in both the primary scanning direction and sub-scanning direction within a predetermined multi-tone range.
In such a printing device allows the direction of primary scanning during the printing of dots by each ink within a predetermined multi-tone range to be locally aligned in both the primary and sub-scanning directions (henceforth referred to simply as xe2x80x9cdot forming directionxe2x80x9d). No shift in the dot forming positions is produced during bi-directional printing in areas where the direction of primary scanning has thus been locally aligned. The printing device suppresses location deviations in regions where images are printed within a predetermined multi-tone range, suppresses roughness of images, and allows smoother printing. Although various specified multi-tone ranges can be set, the preferred range includes intermediate tones with which irregularities in density are less distinguishable. The predetermined multi-tone range is not necessarily set within a continuous range. A low multi-tone and high multi-tone may be set within the predetermined multi-tone range.
Here, intermediate tone refers to a multi-tone within the multi-tone range which can be reproduced by the printer device. Strictly speaking, it does not mean intermediate values. In a bright multi-tone range, that is, in low multi-tone regions, the dot recording density is lower. Shifts in the dot forming positions are thus less distinguishable and have relatively little effect on image quality. Conversely, in a dark multi-tone range, that is, in higher multi-tone regions, the dot recording density is extremely high. Slight shifts in the dot forming positions are thus difficult to distinguish in the form of irregularities in density and the like, and have relatively little effect on image quality. Intermediate tones thus mean the exclusion of such low and high multi-tones, and mean any multi-tone range which has been set for the purpose of improving image quality. In particular, it is possible to target a multi-tone range in which image quality is significantly affected by shifts produced in bi-directional printing within such a multi-tone range. It is also possible to target a multi-tone range frequently used for commonly printed images.
Gray scale ranges in which image quality is significantly affected cannot be strictly defined as a matter of absolute principle, and vary according to the conditions prevailing during printing, such as the printing resolution or dot diameter. In fact, they may be defined as a multi-tone range in which image roughness is easily discerned when images are printed with various changes in the primary scanning and sub-scanning.
The conditions under which the dot forming direction is locally aligned are described using a specific example. FIG. 8 illustrates a plurality of adjacent raster lines formed in the same direction. An eight-raster line segment is depicted here. As illustrated in this figure, the dots of each raster line are formed aligned either during forward or return travel of primary scanning. From the top, raster lines formed during forward travel in primary scanning alternate in groups of 3 with those formed during return travel. As a result, more than 2 dots formed during forward travel near D1 in the figure, including dot D1, are present in the primary and sub-scanning directions. The direction in which all the dots in FIG. 8 are similarly formed is locally aligned in the primary and sub-scanning directions. When shifts in the dot forming positions occur during forward and return travel in such cases, shifts in the dot forming positions can be discerned in the two locations designated G1 and G2, as shown in the figure.
FIG. 9 illustrates adjacent raster lines formed in different directions. An eight-raster line segment is depicted here as in FIG. 8. A look at the dots in D1xe2x80x2 shows that the dot forming direction is aligned in the primary scanning direction. However, dots formed in different directions are adjacent to each other in the sub-scanning direction, and the direction in which they have been formed cannot be considered to be locally aligned. This is true not only of dot D1xe2x80x2, but all the neighboring dots. In such cases, as shown in the figure, shifts in the dot forming position in the primary scanning direction are discernible in a total of 7 locations comprising g1, g2, g3, etc.
Comparison of FIGS. 8 and 9 clearly shows that the local alignment of the dot forming direction would reduce the locations in which shifts in the dot forming direction can be discerned. In cases of adjacent raster lines in the same direction (FIG. 8), the area of regions G1 and G2 where shifts have occurred is greater than the surface area of regions g1 and g2 in FIG. 9. The effect of such differences in surface area on image quality is relatively low, however, in the relatively high resolution printing executed by recent printing devices. By contrast, as shown in FIG. 9, when shifts occur in several locations, the overall image quality becomes grainy, resulting in rougher printed images as a whole. Accordingly, reducing the number of regions where shifts occur can improve the grainy look of images and provide better image quality.
The above examples were of groups of 3 adjacent raster lines formed in the same direction, but there are fewer locations in which shifts can be discerned depending on the number of adjacent raster lines, allowing image quality to be improved. These were also examples in which the dot forming direction was aligned for each raster line, but the direction of formation of all the raster lines does not have to be aligned, as long as the condition stipulating the local alignment of the direction in which adjacent dots are formed is met. For example, the dot forming direction for the raster lines may vary every two raster lines. That is, in the example depicted in FIG. 8, dots D1 and D2 may be formed during forward travel, dots D3 and D4 may be formed during return travel, and dot D5 may be formed during forward travel again.
Conditions stipulating the local alignment of the direction in which adjacent dots are formed can be satisfied in various ways according to the dot printing rate in intermediate tones. FIG. 8 shows an example in which the direction of formation is locally aligned in a case involving a high printing rate of close to 100%. This printing method allows the direction of formation to be locally aligned at a high printing rate of more than 50%. In this case, there is a high possibility that the dots will be formed for adjacent pixels in the primary or sub-scanning direction. Printing in the manner depicted in FIG. 8 therefore allows the direction in which adjacent dots are formed to be locally aligned.
The dot printing rate in intermediate tones will vary according to the conditions during printing. For example, when printing is performed using large-diameter dots, intermediate tones are developed at a low printing rate. FIG. 15 illustrates a plurality of adjacent raster lines formed in the same direction at a low printing rate. In this case, adjacent dots are not necessarily limited to being formed for adjacent pixels in the primary and sub-scanning directions. Here, dots are formed 1 pixel at a time in each direction. When printing is done in the same manner depicted earlier in FIG. 8 at this printing rate, the raster line r1 is formed in a different direction than the direction in which the adjacent raster lines above and below are formed, as shown in FIG. 15. In other words, the direction in which the adjacent dots around raster line r1 are formed is not locally aligned.
FIG. 16, meanwhile, illustrates adjacent raster lines formed in different directions at a low printing rate. When printing is done in the same manner depicted earlier in FIG. 9 at this printing rate, the direction in which the adjacent dots are formed is aligned, as shown in FIG. 16. Thus, in the case of a low printing rate, image quality can be improved by alternating the direction in which the raster lines are formed. Of course, this does not mean that the raster lines formed in different directions in the above examples must be adjacent in an alternating manner. The control parameter stipulating the local alignment of the direction in which adjacent dots are formed can be set in a variety of ways according to the printing rate of the dots representing intermediate tones.
FIGS. 15 and 16 illustrate cases of systematically dispersed dots. In actuality, it is possible that the dots will not be systematically formed at such a printing rate, and that the dots will be formed for adjacent pixels in the primary or sub-scanning direction. Such locations are relatively few, however, and the dots are formed as depicted in FIGS. 15 and 16 when viewed locally. The present invention is intended to provide effects in suppressing roughness by suppressing shifts in the dot forming positions. Accordingly, the conditions under which the dot forming direction is locally aligned do not have to be strictly observed for all image regions, but should be observed for the most part.
The printing device can provide the effects described above, and can also suppress banding because a head having nozzles at intervals of two or more raster lines is used for interlaced format printing. Based on the actions described above, the printing device allows the direction in which the dots are formed to be locally aligned during high speed printing based on bi-directional printing, thereby enabling high image quality printing with less image roughness.
As noted above, the printing device does not necessarily require alignment of dot formation for each raster line, but the control parameters are preferably set in compliance with conditions under which the direction of primary scanning during the formation of dots is aligned during either forward or return travel for each raster line.
When done in this manner, the dot forming direction is aligned for each raster line, thus ensuring that the dot forming direction is aligned in the primary scanning direction. When shifts in the dot forming position occur while dots formed during forward travel and dots formed during return travel are present together in the raster lines, the dot density is quite easily discerned, tending to result in a loss of image quality.
The control parameters of the printing device are also preferably set in compliance with conditions under which the raster lines are formed by a plurality of primary scans.
This allows overlapping format printing to be done, wherein the raster lines are formed by a plurality of primary scans. It is thus possible to disperse the shifts in dot forming positions caused by the ink discharge properties and the like, and to reduce banding. It is thus possible to realize printing with even higher image quality.
As noted previously, the control parameters stipulating local alignment of the direction in which the dots are formed vary according to the printing conditions. The memory thus preferably further storing the control parameters according to the resolution.
When the printing resolution is changed, the diameter of the dots being used is primarily changed, and the dot printing rate in intermediate tones is changed. Thus, as described in the comparison between FIGS. 8 and 9 and FIGS. 15 and 16, the control parameters for performing suitable printing also change. A printing device having the structure allows printing to be carried out with the use of separate control parameters according to resolution, and thus allows high image quality printing to be realized for each resolution. When the resolution is changed, there may also be some changes in multi-tone ranges which are susceptible to readily discernible roughness. In the printing device, the predetermined multi-tone range serving as the basis for setting the control parameters does not necessarily require alignment for each resolution, and may be suitably selected for various resolution levels.
The printing device also preferably includes printing mode setting unit for determining whether or not text images are to be printed; and printing controller for controlling the head drive controller and the sub-scanning mechanism to execute printing based on control parameters only when the text image printing mode has been set.
Such a printing device allows printing to be carried out with control parameters that are different depending on whether or not text images are being printed. Printing based on the control parameters indicated earlier can improve grainy images and can dramatically improve image quality in cases where dots are formed throughout virtually the entire image, such as natural images. That is, this embodiment is suitable for cases in which a non-text image printing mode is set. The printing device enables printing based on the control parameters given above in this mode.
In the printing device, the memory storing text printing control parameters, including the position of the pixels that are to be formed during each primary scan, and the feed of the sub-scan set in compliance with conditions allowing the raster lines to be formed by a plurality of primary scans, and conditions allowing the direction of the primary scan forming the pixels to be aligned during either the forward or return travel for each position in the direction; and the printing controller can execute printing based on text printing control parameters instead of the above control parameters when the text image printing mode has been set.
This allows the image quality of text images to be improved. When printing is based on text printing control parameters, the direction of movement for forming pixels matching the primary scanning direction position is aligned. As a result, it is possible to more accurately represent a variety of straight lines, particularly straight lines in the sub-scanning direction. Text images include an abundance of straight lines. The printing device allows straight lines to be more accurately represented, and can thus improve the image quality of text images. Text images refer images containing text, as well as images containing an abundance of linear and other geometrical patterns, such as graphs.
The memory can store the control parameters according to the type of printing medium.
When the printing medium is changed, the diameter of the dots being used is primarily changed, and the dot printing rate in intermediate tones is changed. Thus, the control parameters for performing suitable printing also change in accordance therewith. Different types of printing media are often designed for a particular intended use. That is, the types of images to be printed are often primarily established for each printing medium. As noted above, the dot formation format for suitable printing varies depending on the type of image. The printing device allows suitable printing to be realized with the use of separate control parameters according to the printing medium by taking such differences into account.
In the printing device, the head should include nozzles arranged in the sub-scanning direction at a predetermined interval of 3 dots or more. Such a head allows a plurality of raster lines formed in the same direction to be adjacent to each other, without significant decreases in the nozzle operating efficiency. Of course, the printing device can be adapted for printing devices equipped with heads comprising a variety of nozzle pitches and number of nozzles, and may also involve the use of heads in which the nozzles are arranged at a pitch of 2 dots, while portions of the nozzle are masked so as to comply with the above conditions.
Increasingly higher resolution levels have been reached in recent printing devices. There is also a general need for printing with higher image quality during such high resolution printing. When greater image quality is to be achieved during such high resolution printing, the printing device includes a printer device for printing multi-colored images by means of primary scanning, in which a head travels back and forth relative to a printing medium to form raster lines comprising rows of dots in the direction, and sub-scanning, in which the printing medium is conveyed relative to the head in the direction perpendicular to the primary scanning direction, so as to form dots for the pixels on the printing medium, the head being a head comprising, in the sub-scanning direction, at intervals of two or more raster lines per color, a plurality of nozzles for discharging ink, wherein the printing device includes: memory for storing control parameters, including the position of the pixels that are to be formed during each primary scan, and the feed of the sub-scan; head drive controller for driving the head while moving back and forth in the primary scanning to form dots for the pixels specified by the control parameters; and sub-scanning mechanism for effecting sub-scanning at a feed specified by the control parameters. The parameters are set in compliance with conditions allowing the direction of the primary scan during the formation of dots to be aligned during either the forward or return travel for each raster line, and conditions under which two or more raster lines formed in each direction of primary scanning are adjacent to each other.
During high resolution printing, there is a high probability of the dots being formed for adjacent pixels in the primary and sub-scanning directions. Printing based on control parameters set under the conditions thus allows the direction in which dots are formed to be locally aligned, and allows higher image quality printing to be achieved with less image roughness.
Printing devices for high resolution printing commonly include a printing mode for rapid, low resolution printing.
The printing device thus preferably further includes resolution setting unit for setting the resolution during printing as a printing condition; and printing controller for controlling the head drive controller and the sub-scanning mechanism to execute printing based on the control parameters when the resolution is no less than a predetermined level.
This allows high image quality printing to be achieved during high resolution printing.
In this case, low resolution printing can be managed in various ways.
The memory should furthermore stores second control parameters, including the position of the pixels that are to be formed during each primary scan, and the feed of the sub-scan; and the printing controller executing printing based on the second control parameters when the resolution is below the predetermined level. The second parameters are set in compliance with conditions under which the raster lines are formed by a plurality of primary scans, conditions allowing the dot forming direction to be aligned during either the forward or return travel for each raster line, and conditions under which raster lines formed during movement in different directions are adjacent to each other.
When high resolution and low resolution printing modes are set within a practical range in printer devices, intermediate tones are often represented at a printing rate of around 25% in low resolution mode. This corresponds to the printing rates in FIGS. 15 and 16 described previously. The printing device thus allows the direction in which adjacent dots are formed to be locally aligned even at low resolution levels, and allows image roughness to be suppressed.
The predetermined level of resolution serving as a basis for changing the particulars of control by the printing controller can be preset in a variety of ways based on the relation between resolution and roughness. The predetermined level can be preset as a constant level, and when the resolution setting unit allows the resolution in the primary scanning direction and the resolution in the sub-scanning direction to be set to different levels, the printing controller may adapt the resolution in the sub-scanning direction to the predetermined level and effect the control according to the relationship of the magnitude between the resolution in the primary scanning direction and the predetermined level.
Here, the resolution in the primary scanning direction and the resolution in the sub-scanning direction is not necessarily to set both to any combination. Based on the predetermined correlation, there are also cases capable of different level settings. For example, xe2x80x9cthe resolution in the primary scanning direction ( the resolution in the sub-scanning directionxe2x80x9d can be set to a predetermined combination such as xe2x80x9c360 dpi (720 dpi, xe2x80x9d xe2x80x9c720 dpi (720 dpi,xe2x80x9d and xe2x80x9c1440 dpi ( 720 dpi,xe2x80x9d so that the resolution levels are set by selecting from these.
The present invention can also further include printer devices in the following embodiments.
That is, a printing device includes: memory for storing control parameters, including the position of the pixels that are to be formed during each primary scan, and the feed of the sub-scan; printing condition inputter for inputting printing conditions; head drive controller for driving the head while moving back and forth in the primary scanning to form dots for the pixels specified by control parameters according to the printing conditions; and sub-scanning mechanism for effecting sub-scanning at a feed specified by the control parameters according to the printing conditions. The parameters are set so that the dots formed in the same primary scanning direction are formed adjacent to each other according to the printing conditions.
In this printing device, the printing conditions can include the resolution during printing, and the control parameters are at least set so that the number of dots formed in the same primary scanning direction, which are adjacent to each other in the sub-scanning direction, is a value corresponding to the resolution.
The printing conditions can also include the type of images, and the control parameters can be at least set so that dots formed in the same primary scanning direction are aligned with pixels of the same position in the primary scanning direction, for text images.
The printing conditions can also include types of printing media.
In this case, printing media primarily used for printing text images, for example, are printed in the same manner as text images, and printing media primarily used for natural image printing are printed while conditions are set so that a plurality of dots formed by primary scanning in the same direction are adjacent to each other in the sub-scanning direction. The printing mode for each printing medium may be set in consideration of the diameter of the dots that are formed, irrespective of the intended use of the printing medium.
The printing device of the present invention can be adapted for heads discharging ink in a variety of ways. Methods that can be adapted include, for example, the use of electrostrictive elements such as piezo elements to alter the ink channel in the nozzle and discharging ink with the application of pressure. Another method that can be adapted is to apply electricity to a heater inside the ink channel to produce gas bubbles in the ink, so as to make use of the pressure of the gas bubbles to discharge the ink.
The present invention can be constructed as a printing method in addition to the structure of the printing devices. The method can be realized in a variety of ways, such as a computer program for executing the printing method or printing device, recording media on which such a program is recorded, and data embodied in carrier waves, including such programs. It also goes without saying that various added elements indicated in the printing devices above can be adapted in various ways.
When the present invention includes a computer program, or recording media on which such a program is stored, or the like, the invention may include the entire program for operating the printing device or only those portions enacting the functions of the present invention. Examples of recording media which can be used include floppy discs, CD-ROM, opticomagnetic discs, IC cards, ROM cartridges, punching cards, bar codes, and other printed materials with codes printed thereon, and various media which can be read by computers such as internal memory devices of computers (memory such as RAM or ROM) and external memory devices.